1. Field of the Invention
The invention concerns an electrically insulating substrate, made of glass notably, the valuable feature of this substrate being that it be made with a large area, and in a simple and economical fashion. The substrate according to the invention can be advantageously applied, for example, to the field of liquid crystal display panels or, again, to the field of devices coming under thin film technology such as array type or surface type solid state photosensitive devices or, again, surface structures of hydrogenated amorphous silicon carbide based electroluminescent diodes.
2 Description of the Prior Art
In these different fields, the making of large area substrates raise problems of varying acuteness, depending on the field concerned.
If we take, for example, the case of a surface type image detector, formed by a matrix arrangement of photosensitive dots made with amorphous silicon according to thin film technology, such an image detector usually has to be large in size, with dimensions of up to 40 cm.times.40 cm for example. The standard practice in this type of application is to use glass substrates that are free of alkaline constituents in their composition. One of the most common substrates of this kind is a barium borosilicate marketed by the firm CORNING under the reference 7059.
This type of glass has drawbacks that make it difficult to use it in industrial conditions. For, it is a hard glass and is, consequently, very brittle at the edges and gets easily flaked. The particles that thus get detached go on to the surface and scratch it and/or tend to alter or disturb the active structure (forming the detector) that is deposited on the glass. This leads to a great many localized faults. Moreover, as this glass is very hard, it is very difficult to cut it, and it often gets broken when it has to be cut into sections with large areas, notably of 40 cm.times.40 cm. This tends to increase costs, whereas the purchase of this glass already entails a very high cost in view of the criteria of planeity that are required.
However, there are soda-lime type glasses called window glasses which are easy to cut and widely available in the market. But this type of glass has the drawback of being rich in sodium oxide. If, for example, amorphous silicon is deposited on a support or substrate made of glass of a such a type, the alkaline constituents present in the glass get diffused in the amorphous silicon and, consequently, the active structure attached to this substrate ages poorly and gets detached from its support after a few months.
In a standard approach to this problem, the glass is passivated by depositing a protective layer, called a barrier layer, on its surface. This barrier layer is thus interposed between the glass and the first layer of the active structure for which the glass acts as a support. The function of the barrier layer is to act as a barrier to the undesirable products contained in the glass support, i.e., in this case, its function is to prevent the diffusion of the alkaline constituents in the amorphous silicon.
In this spirit, it is possible to use soda-lime glass passivated with silica (the silica layer being generally deposited in using a liquid phase of a metal-organic silicon compound). However, the substrate made of soda-lime glass treated in this way is made only in very small quantities and it is consequently very difficult to procure supplies of it for industrial-scale manufacture.